Capillary wave properties of a spread film of a polybutadiene-poly(ethylene oxide) block copolymer: 2. Cyclohexane-water interface

A diblock copolymer of polybutadiene and poly(ethylene oxide) has been spre ad at the interface between cyclohexane and water, and the properties of th e capillary waves were determined by light scattering. As the concentration of block copolymer at the interface increases, the capillary wave frequenc y and damping decrease at a rapid rate initially but tend to asymptotic val ues at interfacial concentrations of 2 mg m(-2) and greater. Analysis of th e correlation functions of the scattered light for the interfacial viscoela stic moduli (interfacial tension, dilational modulus, and dilational viscos ity) required the incorporation of the dilational terms to obtain a fit to the data that displays uncorrelated residuals. Interfacial tensions obtaine d by light scattering were in reasonable agreement with those obtained from interfacial pressure isotherms and exhibited the same dependence on the in terfacial concentration of the copolymer. The dilational moduli from the li ght scattering data were in rather poor agreement with the values from isot herm data, and this is attributed to the low sensitivity of the light scatt ering data to the dilational modulus because of the reduced coupling at liq uid-liquid interfaces. The dilational viscosity however had the same charac teristics that have been observed for graft copolymers at the air-water int erface, and a Kramers-Kronig type relation has been invoked to explain the observations in a qualitative manner. The frequency dependence at fixed cop olymer concentration of the interfacial tension and dilational modulus disp lays no characteristic behavior, but the dilational viscosity shows an extr emely regular dependence on the capillary wave frequency. The behavior can be explained in a qualitative way by. invoking relations used for surfactan t solutions where a barrier to surface adsorption is postulated.